How to supply, load, and test power-management circuits (Part 1 of 2)

Even though such boards are easy to use, some additional knowledge is very important. How should power be supplied to an evaluation board, and how should it be loaded? This question may sound simple, but the effect of the lab power supply, and the leads going to the board under test, as well as the type of load which is being used and how it is connected, are very important things to consider. Only with a good understanding of the measurement setup can a design engineer really use an evaluation board or any test board he or she might have.

This article deals with the effect of using different lab power supplies, as well as different turn-on methods including relay turn-on, solid-state switch turn-on, and plugging in cables for turn-on. Different methods are explained and compared with each other. The article will explain the effect of an electronic load versus a resistive load for testing purposes. Especially for evaluations of soft-start and short-circuit recovery, there are significant differences in the different loading types available.

A typical lab setup
A typical lab setup is shown in Figure 1. It consists of the dc/dc converter (the evaluation board) under test in the middle, and it has cable connections to the power source (lab power supply) and to the load (passive resistive load or electronic load).

Figure 1: Lab setup to test a power-supply evaluation board
(Click on image to enlarge)

Figure 2 shows the electrical equivalent circuit of such a basic setup.

Figure 2: Electrical equivalent circuit of a lab power supply
(Click on image to enlarge)

The lab power supply acts very much like a typical voltage source. There is an ideal voltage source which has some source impedance (Z1). The cables supplying the evaluation board (circuit under test) have some inductance as well as some capacitance. The capacitance is not shown explicitly in Figure 2 since the effect of the capacitance is very small. Usually, the standard lab cables with banana plugs come in individual lines (red and black) and are not attached together. This makes the capacitance very small.

The inductance, however, is quite significant. Inductance in an ac circuit becomes higher as the area which the electric-current flow encloses becomes larger. Having the plus (positive) as well as the minus (negative) cable close together will reduce the inductance of the supply cable. The significant attribute of the device under test is the input capacitor, as well as the output capacitor.

Power-management evaluation boards typically have input and output capacitors, since they want a good voltage source as input and they want to act like a voltage source on the output. The impedance Z2 is the dc/dc converter impedance. It has some impedance to ground which represents all the power-supply losses, and it has an input impedance as well as an output impedance. For simplicity, the complete internal impedance is referred to as Z2.

The load cable would attach the output of the circuit under test to a load. The load has an input impedance, Z3, which can vary from being purely resistive to inductive, capacitive, or combinations.

Powering the circuit under test
There are different ways of powering a circuit under test. Here are two of the most common ways and a short evaluation:

1) Turning on the lab power supply and plugging in the supply cable into the lab power supply. This method is similar with putting a relay (mechanical switch) or a power transistor into the power path.

The problem with this approach is that input capacitance of the circuit under test is not charged. As soon as the voltage on the lab power supply is attached to the supply cable, the potential difference between the lab power-supply output and the circuit under test input is Vout of the lab power supply. The inductance of the supply cable will increase the current, up to the point when the input capacitors of the circuit under test are fully charged up. When the voltage across these capacitors is the same as the output voltage of the lab power supply, the current through the supply cable should stop or limit to the current Z2 is asking for.

The inductance of the supply cable prevents instantaneous current changes, and as a result of this, the energy stored in the supply-cable inductance forces the voltage across the input capacitor of the circuit under test to overshoot. The amount of overshoot depends highly on the damping factor of the LC system. The capacitor actually has some ESR (equivalent series resistance) which helps the damping of the voltage overshoot. With very-low ESR capacitors such as ceramic capacitors, there is very low damping, causing the potential overshoots to be larger. The maximum amount of overshoot that can ever be seen across the input capacitors is the lab power-supply output voltage times two.

Figure 3 shows the input voltage of a LM2676 evaluation board with only ceramic input capacitors, when it is suddenly powered using a supply cable two feet in length by attaching the 'hot' supply cable to the evaluation board.

Figure 3: Input-node voltage overshoot
(Click on image to enlarge)

The lab power supply is set to 20 V but the voltage peak which can be seen after Cin is fully charged is 34 V. Since the LM2676 is rated for input voltages up to 40 V, our circuit under test did not get damaged.

If we had used an evaluation board with a lower maximum input-voltage rating, or if we had done the same test with a higher nominal-input voltage, this voltage spike on Vin can easily damage the switching regulator on the evaluation board, as well as the input capacitors.

A soft-start function of the device under test can not prevent this overshoot, since the soft-start only controls the behavior of Z2 and cannot influence the input capacitor. This situation also exists for enable functions of the device under test. In addition to the drawback of the voltage overshoot, hot plugging of devices under test may cause dc-arcing at higher-power tests, and some bouncing may occur when plugging the devices in by hand.

2) Powering up the attached circuit under test with the lab power-supply output-enable button. This way of powering circuits is usually a good way of testing power-supply boards. This output-enable function usually has a soft-start functionality built into it and brings up the supply voltage slowly. However, some power supplies do not have an output-enable button. In this case, the best idea is to set the lab power supply to 0 V, attach the circuit under test and then to increase the lab power supply voltage by hand to the nominal supply voltage.

The lab power supply usually has an adjustable current limit with a maximum setting, as well as a maximum power limit. It is very important to understand that during startup not only is the final load supplied, but also the input and output capacitors of the device under test are charged up. Depending on the maximum rating and the current fold-back behavior of the lab power supply, the circuit under test might not get powered up at all. If the lab power supply is dimensioned so that the maximum power or current capability is somewhat close to the nominal, steady-state power consumption of the final load, it might be necessary to separate the different energy consumers during startup.

These energy consumers include:
a) The input capacitor of the device under test. Only the lab power supply's soft-start function, or cranking the supply up by hand, can influence this.
b) The output capacitor of the device under test. This load can be minimized during startup if the device under test converter has a soft-start function.
c) The final load. It can be turned on after the device under test is up and running with charged-up output capacitors.

The most challenging situation for the lab power supply is when the device under test has large input capacitors, has a very short or no soft-start, has large output capacitors, and the final load is attached and acts capacitive. In such a startup situation, the lab power supply will have to be capable of handling multiple times the nominal, steady-state power of the setup. If the lab power supply can not handle this power, the result may be that the device under test will not start up at all, or in borderline cases the power supply will only come up after some time.

(Note: Part 2 of this article will examine the different concerns with different topologies, loading the circuit under test, and if lab testing represents reality. You can read it by clicking here.)

About the author
Frederik Dostal is an Application Engineer for Power Management at the National Semiconductor Corp. Design Center in Phoenix, Arizona. He joined the company in 2001, supporting Europe. After two years, he became a Field Application Engineer for Central Europe, covering many automotive accounts. His current position involves product development as well as support for switching regulators, controllers, and linear regulators. Frederik holds an Electrical Engineering Diploma (Dipl.-Ing.) from the Friedrich-Alexander-Universität in Erlangen, and is a member of the IEEE.